Single-molecule surface-enhanced Raman spectroscopy of nonresonant molecules.

Single-molecule surface-enhanced Raman scattering (SERS) detection of nonresonant molecules is demonstrated experimentally using the bianalyte SERS method. To this end, bianalyte SERS is performed at 633 nm excitation using the nonresonant molecule 1,2-di-(4-pyridyl)-ethylene (BPE) in combination with a benzotriazole derivative as a partner. The results are then extended to the even more challenging case of a small nonresonant molecule, adenine, using an isotopically substituted adenine as bianalyte SERS partners. In addition, SERS cross sections of single-molecule events are quantified, thus providing estimates of the enhancement factors needed to see them. It turns out that an enhancement factor on the order of approximately 5 x 10(9) was sufficient for single-molecule detection of BPE, while maximum enhancement factors of approximately 5 x 10(10) were observed in extreme cases. In the case of adenine, single-molecule detection was only possible in the rare cases with enhancement factors of approximately 10(11). This study constitutes a quantitative fundamental test into the lowest detection limits (in terms of differential cross sections) for single-molecule SERS.